.demo {
    position: absolute;
    /* 绝对定位会脱离body */
    left: 0;
    top: 0;
    border: 2px solid red;
    width: 100px;
    height: 100px;
    background: brown;
}

body {
    margin: 0;
    height: 100vh;
    box-sizing: border-box;
}